Vehicle air conditioning systems include a compressor that compresses and superheats refrigerant vapor, which then runs through a condenser, expander and evaporator in turn before returning to the compressor to begin the cycle again. The output of the evaporator includes more than just refrigerant, carrying a component of lubricating oil and some small amount of water, all three of which are in a vapor-liquid mixture. Interposed between the evaporator and compressor is a so called accumulator, also known as an accumulator dehydrator or simply A/D, which is designed to accomplish several objectives. Primarily, the so called A/D operates as its name would suggest, receiving and accumulating the evaporator output and serving as a reservoir or separator in which liquid collects at the bottom and vapor at the top. A return tube internal to the canister has an open inlet located near the canister top end. The canister suction line is connected to an outlet end of the return tube, so that the compressor draws primarily vapor, through the return tube inlet. However, the accumulated liquid also has to be drawn out, since it will not vaporize quickly enough to simply be drawn out from the vapor space at the top of the canister. To draw down the accumulated liquid, a bleed hole assembly, consisting of a small hole in the return tube and a surrounding filter screen, is located on the return tube, near the canister bottom end. As suction is applied to the return tube, accumulated liquid, both the refrigerant and oil component, is aspirated through the bleed hole.
The shape of the return tube varies depending on where its outlet can be located. Ideally, the return tube outlet can be placed high in the canister, near the upper end. This allows the return tube to have a U shape, with the bleed hole assembly located at the bottom bend in the U. Then, there is a built in trap to prevent accumulated liquid from draining down into the compressor suction line when the compressor is off for extended periods. This can cause so called "slugging," undesirable noise when the compressor is turned on again. Sometimes, under hood packaging and line routing considerations dictate that the return tube be a single straight length of tube that runs directly out the bottom end of the canister. In that case, it has been necessary to put a so called J bend in the compressor suction line itself, external to the canister, to act as a trap to prevent leak down. There are circumstances where there is no room for an external J trap, either. In any case, an internal leak down prevention mechanism would be simpler to install in cases where a bottom outlet canister type A/D was required. Another consideration in canister type A/D design is water vapor separation. The canister represents a convenient location for bags of water vapor desiccant. Many simple A/D designs, that is, those that use simple bags of desiccant, only have room for the bags at or near the canister bottom, right within the collected liquid pool, and often require a separate fastener to hold the bag or bags in place.